Two types of conventional electron multipliers are routinely used. A first type, pictorially illustrated in FIG. 1, consists of discrete dynode multipliers, which comprise dynodes stages 10 that initiate and amplify a cascade of electrons. U.S. Pat. No. 4,668,890, issued May 26, 1987, details this type of electron multiplier. Typically, dynode stages 10 are biased using resistor divider string 20 such that front dynode 12 of the multiplier is biased to a high negative voltage (e.g., several kilovolts) relative to last dynode 14 and anode 16 of the multiplier. Thus, an electric field is imposed between each of the dynodes. As incoming particle 30 strikes the front dynode 12 it generates an average of γI secondary electrons 32 from the impact surface of front dynode 12. These secondary electrons are accelerated by the imposed electric field toward the next successive dynode, where they impact and generate more secondary electrons. This cascade of electrons continues throughout the entire series of dynode stages with the cumulative charge of the electron avalanche growing at each stage. After last dynode 14, the electron avalanche charge is collected on anode 16.
The gain (GD) of a discrete dynode multiplier, which equals the cumulative output electron charge per incident particle, corresponds to:GD=γIγSEN−1  (Equation 1)where γSE equals average number of secondary electrons emitted by an electron from one dynode impacting on the next sequential dynode and N equals the number of dynodes used in the detector. To maximize the gain, the dynode material is often selected for high secondary electron emission yield (γSE) properties (See U.S. Pat. No. 5,680,008, issued Oct. 21, 1997).
The second type of multiplier is a continuous electron multiplier, pictorially illustrated in FIG. 2. Channel electron multipliers and microchannel plate (MPC) detectors are specific examples of this type. MPCs employ one or more high resistivity glass channels or tubes 40, each of which acts as a series of continuous dynodes. Patented examples of this type of electron multiplier include: U.S. Pat. No. 4,095,132, issued Jun. 13, 1978; U.S. Pat. No. 4,073,989, issued Feb. 14, 1978; U.S. Pat. No. 5,086,248, issued Feb. 4, 1992; U.S. Pat. No. 6,015,588, issued Jan. 18, 2000; and U.S. Pat. No. 6,045,677, issued Apr. 4, 2000.
As with the discrete dynode, channel front 42 is negatively biased several kilovolts relative to the channel back 44 and anode 50, so that an electric field is imposed inside of the channel from the front (entrance) to the rear (exit). Incident particle 60 impacts channel front 42 and generates secondary electrons 62, which are then accelerated further into tube 40 by the imposed electric field. Secondary electrons 62 impact channel wall 41 and generate even more secondary electrons. The cumulative charge of the electron avalanche grows as it traverses tube 40. The avalanche of secondary electrons 62 exits tube 40, and is collected on anode 70. The gain of a continuous electron multiplier can be modeled as a series of discrete dynodes and can therefore be represented by Equation 1. A variation of this concept uses a porous media having irregular channels; e.g., U.S. Pat. No. 6,455,987, issued Sep. 24, 2002.
A foil electron multiplier, in accordance with the present invention, encompasses the next generation design of electron multipliers. In a preferred embodiment, a series of extremely thin, in-line foils are used to create secondary electrons. The in-line orientation of the foils coupled with their thinness not only creates secondary electrons, but allows the incident primary particles, and the secondary electrons generated by the primary particles, to continue to the next and subsequent foils. It is believed that this design not only creates a larger avalanche of electrons when compared to historical designs, but also allows for obtaining position-sensitive information on where an incident particle impacted the first stage of the foil electron multiplier. The ability to provide position-sensitive information enables improvements on articles such as flat television screens, computer screens, night vision devices, and the like.
Advantages of the foil electron multiplier design over other types of electron multipliers include:
(1) A higher gain per multiplication stage that results in an increased multiplication efficiency since fewer stages are required to obtain the same charge as other multipliers.
(2) Simplicity of fabrication, since the foil fabrication process (evaporation of a foil material onto a glass slide covered with a surfactant and a subsequent aqueous transfer to a support grid or aperture plate) is simpler than fabrication of continuous multipliers, such as MCPs. The MCP fabrication process requires high purity materials, high precision, a high level of cleanliness, and involves using cladded fibers that must be bundled, stretched, and sintered in cycles, and then cut, etched, and chemically activated.
(3) A lower cost of fabrication, as the fabrication process complexity is reflected in the relevant cost. Twenty commercial foils cost about $500 whereas MCP detectors cost about $5,000 to $10,000.
(4) An ability to cover a larger area, as foils can be evaporated over large surface areas, whereas MCPs require additional bundling and sintering to increase the surface area. Also, large area foils are much more robust as they can be dropped without breaking, whereas MCPs shatter.
(5) Finally, the foil electron multiplier exhibits an intrinsic rejection of ion feedback at each stage. Continuous electron multipliers require a curved or zigzag path to prevent ions from being accelerated back toward the entrance where they can initiate a second pulse. In the foil electron multiplier, ions generated at one foil may be accelerated back to the previous foil, but cannot be re-transmitted back because the ion energy is too low. Therefore, ions can only reach one stage back, and a pulse that they generate will be indistinguishable from the main pulse.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.